1. Field of the Invention
This invention relates to a hollow fiber-type artificial lung used in extracorporeal circulation to remove carbon dioxide from blood and add oxygen to the blood. The invention is applicable to an artificial lung having a blood reservoir chamber and artificial lung having a heat exchanger.
2. Description of the Prior Art
Artificial lungs are broadly classified into those of porous type and membrane type. The membrane artificial lung, such as of stacked membrane type, coil type or hollow fiber type, is widely recognized as being superior to the porous-type artificial lung in view of the fact that the blood conveyed through the lung undergoes less hemolysis, albumin degeneration, clotting and affixation, and as being extremely close to the human lung in terms of its operating mechanism. Nevertheless, because the membrane-type artificial lung possesses a number of disadvantages set forth hereinbelow, the artificial lung of porous type is the one used most widely in open-heart surgery at the present time.
In order to obtain sufficient oxygenation with the membrane-type artificial lung currently available, it is required that the blood flow layer be reduced in thickness. This means a narrow blood flow passage and, hence, a large flow passage resistance. In consequence, it is not possible to achieve perfusion of the blood within the artificial lung by utilizing the head developed between the patient and the lung. Accordingly, as shown in FIG. 1, a blood circuit using a membrane-type artificial lung requires that a pump 2 be disposed on the inlet or venous side of the artificial lung, indicated at 1. A blood reservoir 3 and a heat exchanger 4 are also provided. With the blood circuit shown in FIG. 1, however, the magnitude of the pressure adjacent the outlet of the pump 2 is greater than the sum of the pressure loss at the blood feeding catheter and the pressure loss of the artificial lung. The problem that results is an increase in the internal pressure of the circuit on the blood feeding side. A proposed solution to this problem, disclosed in the specification of Japanese Patent Application Laid-Open No. 50-9299, is to pass the blood on the outer side of the hollow fibers. However, the proposed arrangement has not been put into practical use due to difficulties in removing air bubbles developed in the blood in the extracorporeal circuit. Further, there are difficulties in priming and the like in placing the proposed artificial lung into practical use.
The specification of the abovementioned publication discloses a theoretical arrangement for passing oxygen gas on the outer side of hollow fibers, but the arrangement noes not maximize the gas exchange capability of the hollow fibers. To obtain a practical system, not only must the gas exchange capability be improved, but the following factors must be taken into consideration. Specifically, through use of the blood reservoir 3 shown in FIG. 1, the extracorporeally circulating blood is temporarily stored so that any air bubbles entrained within the blood may be removed. The reservoir 3 is also necessary for the purpose of maintaining a certain degree of blood flow in the event that the blood extracted from a vein is deficient because of a bend in the associated tubing, or if there is leakage of blood from the system. However, since, the blood reservoir 3 is provided in the blood circuit independently of the artificial lung 1 in the conventional membrane-type artificial lung system, the circuit is structurally complex and much time and effort are involved in setting up the circuit and in extracting bubbles during priming. Furthermore, because of the extensive priming and the large amount of blood required to fill the conventional system, it is required that a preliminary transfusion of blood be made into the priming liquid, with which the artificial lung is filled in advance, in order to mitigate dilution of the blood within the patient's body. In particular, the allowable amount of blood available for filling an artificial lung for surgery involving infants and children is small because of low body weight. Therefore, when the membrane-type artificial lung, which requires a large quantity of blood to fill the entire circuit, is used in surgical operations on infants or children, a problem arises in that the total amount of blood available is small.
The heat exchanger 4 in the blood circuit of FIG. 1 is needed for lowering blood temperature during a low body temperature process, and for heating the blood or for keeping the blood warm. However, since the heat exchanger 4, as well as the blood reservoir 3, is provided in the blood circuit independently of the artificial lung 1 in the conventional membrane-type artificial lung system, the circuit becomes even more complex structurally and greater time and effort are required for circuit set up and bubble extraction during priming. Also, as mentioned above, the extensive priming and the large amount of blood required to fill the conventional system require that a preliminary transfusion be made in the priming liquid, with which the artificial lung is filled in advance, to counter dilution of the blood within the patient's body. Because of the small amount of blood available for filling an artificial lung in surgery directed to infants and children, there is a demand for an arrangement capable of greatly diminishing the amount of blood needed to fill the overall blood circuit.